In a CDMA mobile communication system, a terminal and a base station interchange information by establishing a radio link between them. The link from the base station to the terminal is called a forward link, and the link from the terminal to the base station is called a reverse link. In the reverse link, if all users in the cell transmit signals with the same power, the signal from the terminal near to the base station is strong when reaching the base station, while the signal from the terminal far away from the base station is weak when reaching the base station, and the weak signal will be submerged in the strong signal so as to cause the problem of “near-far effect”. The CDMA system is a self-interference system in which all users use the same frequency, and if the transmitting power of a user is strong, it will generate interference to other users and affect the system capacity. Similarly, if the transmitting power of a user is weak, it will make its communication quality poor or its access success rate low. Hence in order to overcome the problem of near-far effect and guarantee the communication quality of all users and the system capacity, the technology of reverse power control is applied in the CDMA system.
Reverse power control includes reverse open loop power control, reverse close loop power control and reverse outer loop power control. In the reverse open loop power control, the terminal estimates the quality of the radio environment of the reverse link according to the received power strength of the forward channel, and then determines the reverse transmitting power. The reverse open loop power control is based on the assumption that the deterioration of the reverse link is similar to that of the forward link.
The reverse close loop power control is implemented by the terminal together with the base station. The base station detects the Eb/Nt of the received reverse channel (Eb/Nt is the ratio of the energy received per bit to the total effective noise power; it is a parameter reflecting the signal to noise ratio), and compares the detected measurement value with the reverse outer loop threshold, so as to generate a power control command to the terminal. When the measurement value is larger than the reverse outer loop threshold, the base station instructs the terminal to reduce the transmitting power; and when the measurement value is smaller than the reverse outer loop threshold, the base station instructs the terminal to increase the transmitting power.
The reverse outer loop power control is a process that the base station adjusts the reverse outer loop threshold according to the frame error ratio (FER) of the reverse service channel; this process is implemented at the base station side. In the process of the reverse outer loop power control, a select distribute unit (SDU) calculates the current FER according to a power measurement report of the reverse service channel, compares the current FER with a target FER, and then dynamically adjusts the reverse outer loop threshold value according to the comparison result.
When a terminal accesses a base station system, the reverse open loop power control is firstly used; and when a service channel is established successfully, and the base station receives the prefix of the cell phone but not yet activate the select distribute module (SDM), whose function is to forward frames to an audio or data processing module according to the types of the frames, the reverse close loop power control is used, wherein the base station sets the same reverse outer loop threshold value for terminals of different radio configuration types. In a CDMA2000 system, it may be needed to support terminals of RC1 and RC3, since CDMA terminals use different RCs, different data transmission rates, and different transmitting powers. If the set reverse outer loop threshold value is low, it will cause the transmitting power of terminals of some RC types (such as RC1) to be always very low (the transmitting power can not be higher than the reverse outer loop threshold value), consequently, the base station cannot activate the SDM all the time and the access fails at last. Such situation may not happen to terminals of some RC types (such as RC3), but may happen to terminals of other RC types (such as RC1).